The present invention pertains to a process for the polymerization of ethylene for obtaining a polymer with a broad molecular-weight distribution. Specifically, the objective of the invention is to obtain high and low density linear polyethylene. The result is obtained due to the particular treatment, prior to its use with a cocatalyst in the polymerization of the ethylene, of the catalytic component. Prior to its use, the catalytic component comprising at least one magnesium derivative and a chlorine-containing derivative of titanium mainly at the oxidation state 3 and/or 4 undergoes a reduction by a metallic compound possessing at least one metal-carbon or metal-hydrogen bond, followed by a treatment by a transition metal halogen compound.
The invention also pertains to the process for treating the catalytic component.
As used herein the phrase "polymerization of ethylene" means not only the homopolymerization of the ethylene, but also the copolymerization of the ethylene with an alpha-olefin, such as propylene, 1-butene or 1-hexene.
The polymers with a broad molecular-weight distribution, industrially employed in particular in extrusion-blow molding techniques, are distinguished by their polydispersity and their fluidity index from the polymers with a narrow molecular-weight distribution, industrially employed, in particular, for injection molding.
The polymers with a narrow molecular-weight distribution possess, on an average, a polydispersity of about 4 to 6, the polydispersity being the ratio of the molar weight by weight to the molar weight by number. These polymers with high fluidity posses a fluidity index ratio MFR.sub.5-2 less than 3.3, MFR.sub.5` being, according to the ASTM standard D1238, the MI.sub.5 /MI.sub.2 ratio of the fluidity index under 5 kg to the fluidity index under 2.16 kg; the MFR.sub.21-5 ratio of the fluidity indices under 21.6 kg to the fluidity index under 5 Kg, MI.sub.21 /MI5 according to the ASTM standard D 1238, is less than 10. These products are obtained in a single reactor by the polymerization of the ethylene in suspension in solution, or in the gaseous phase in the presence of a specific Ziegler-type catalyst comprising a cocatalyst, in general an alkylaluminum, and a catalytic component containing Ti, Mg, Cl and possibly an electron donor. The products obtained with a narrow distribution possess a limited elasticity which avoids the very negative phenomenon of injection shrinkage.
Due to their lack of elasticity, these products are unsuitable for techniques requiring a high mechanical resistance in the melted state, as, for example, in the case of extrusion-blow molding. When these properties are in demand, one employs polymers with a broad molecular-weight distribution, preferably possessing a fluidity index ratio MFR.sub.21-5 greater than 16 for a fluidity index MI.sub.5 of about 1 to 1.5, or a MI.sub.5 /MI.sub.2 ratio greater than 3.5 for MI.sub.2 &gt;1.
The industrial manufacture of these products in a single reactor presents great difficulties in the presence of a Ziegler-type .catalyst.
According to Zucchini; U. and G. Cecchin: "Control of Molecular-Weight Distribution in Polyolefins Synthesized with Ziegler-Natta Catalyst Systems," Adv. in Polymer Science 51, 101-153 (1983), a document which reflects the prior art in this matter, the best means for obtaining a polymer with a broad molecular-weight distribution in the presence of a Ziegler-type catalyst is to carry out the polymerization in several stages or in a series using at least two successive reactors. However, even under these best conditions, it is not easy to manufacture a polyethylene with a MFR .sub.21-5 greater than 16, a necessary condition being to proceed with catalysts that yield broad distributions in a single reactor. Moreover, this process presents the disadvantage or requiring at least two reactors which leads to a loss in productivity with regard to the significance of the installation and a delicate control due to the activity of several reactors instead of just one.
According to FR-A-2,596,398, it is possible to obtain, by the polymerization of ethylene in a single reactor, a polymer with a broad molecular-weight distribution with a MFR MI.sub.21 /MI.sub.5 greater than 16. obtain this result, a mixture of MgCl.sub.2 and TiCl.sub.4 obtained by joint pulverization is employed as the catalytic component. In addition to the joint pulverization of the components, which requires industrially complex rules, the process presents the disadvantage of using a component with a poorly defined structure, which leads to the manufacture of a polymer with a heterogeneous granular distribution.